Download - Maize

mycotoxin monitoring capacity of regulatory agencies and the private sector, and to add further value
by linking farmers producing mycotoxin‐safe maize with markets.
Lessons learned, challenges to progress, and strategies for success
Significant progress has been made in identifying resistance to ear mold fungus, storage insect pests and
mycotoxin reduction, and tolerance to drought. However, resistance and/or tolerance to these traits is
quantitative and controlled by many genes with small effects (quantitative trait loci—QTL) that are
influenced by the environment. Therefore, progress towards development of varieties/hybrids with high
levels of genetic resistance has been difficult to achieve (Munkvold 2003). This is because breeding
strategies to select for preferred genetic traits (resistance to insects and ear mold) are plagued by many
hurdles including: inconsistent, labor‐intensive inoculation techniques (Campbell and White 1994); lack
of single genes and resistant control genotypes; the cost of evaluating large numbers of progeny,
especially for mycotoxins (Munkvold 2003). Identification of QTL with large effects and development of
gene‐based markers will help circumvent some of these problems and significantly assist product
development. More highly polygenic forms of resistance will require improved predictions of breeding
value using high‐density genomic selection approaches.
Traditionally, breeding programs have used resistance to ear mold as an indication of reduced
mycotoxin accumulation, but a growing body of research is revealing that resistance to ear rots might
not be a very good indicator of resistance to mycotoxin accumulation. QTL mapping is revealing that the
two traits might be under distinct genetic control (Busboom and White 2004; Wisser et al. 2006). The
lack of a cost‐effective mycotoxin assaying tool for routine use in breeding programs has slowed
development of resistant germplasm. However, ICRISAT and IITA have recently developed an ELISAbased
mycotoxin assaying system that reduces the cost of mycotoxin detection and quantification from
approximately USD 15.00 to about USD 1.00 per sample, making it feasible to implement mycotoxin
assaying in regular breeding programs.
Plant stress predisposes maize kernels to colonization and infection by ear rot fungi. High aflatoxin levels
are often associated with abiotic stresses such as drought, heat, and nitrogen‐deficient soils, also with
tillage operations and with biotic stresses such as insects, diseases, and weeds (Moreno and Kang 1999).
A strategy to minimize stress to the plant will significantly reduce infection by ear mold fungi and
subsequent mycotoxin accumulation. Sources of resistance or tolerance to the different stress‐inducing
factors have been identified and some are now available in elite germplasm. The challenge is to combine
the different traits in the same background and test their performance to manage storage insect pests
and minimize mycotoxin accumulation. The doubled haploid technique now being used in CIMMYT will
help rapidly develop lines containing a combination of alleles favorable to different stress factors.
Apart from reducing yield and quality, storage insect pests serve as vectors for grain storage molds,
creating wounds that serve as entry points for the fungus and through respiration, conditions conducive
to fungal growth. CIMMYT has developed cheap technologies to select for resistance to post‐harvest
insect pests and has identified then transferred into elite lines resistance to storage pests. Doubled
haploid technology will be used to combine resistance to storage fungi and insects, so as to minimize
post‐harvest losses.
Adequate storage facilities are essential to preserve quality, minimize storage losses, and maintain food
safety for food security. Although substantial research‐for‐development efforts have gone into storage,
there have been many cases where small‐scale farmers have not taken up the improved storage
technologies —sometimes the technologies turn out to be inappropriate for farmer needs, and they
128